15W-50 vs 20W-40 oils

Question

Which is the thinner oil? 15W-50 or 20W-40

Answer 1

csnubl explained this excellently. One little bit of clarification however...it's not only ambient temperature you need to consider but does your bike generally like to run hot or cooler? (relatively speaking)

Answer 2

The 15w-50 has a range of viscous properties from that of a 15 weight oil to that of a 50 weight oil.  What this means is that it will provide penetrating lubrication at low temperatures as well as a straight 15 weight, while maintaining a lubricating film at high temperatures as well as a straight 50 weight oil.  The 20w-40, well, you get the idea.  So, it would appear that the wider range (15w-50) is better.  However, there are some compromises inherent in providing the wide range of coverage.  The best idea is to use a multi-grade oil that covers your realistic needs, and no more, determined by the temperature ranges expected between oil changes.

Answer 3

thinner? or lower viscosity?

15-50 when it's cold

20-40 when it's hot